The present invention relates to a sector antenna as well as to antenna control in a radio communication system. More particularly, it is an object of the present invention to reduce the interference in the same frequency by correlating a direction of an antenna to an allocated slot position based on a TDMA system used as a multiple access system.
FIG. 14 shows a cellular structure of a radio communication system. Numerals from b1 to b12 indicate numbers allocated to base stations, and base stations b1, b5, b7 and b12 indicate that the base stations use radio waves at the same frequency. In cellular radio communications, a method is generally employed in which a geographic frequency is reused by repeatedly using the frequency in radio zones provided at a specified distance apart from each other so that utilization efficiency of a frequency can be improved. The zones (or cells, or base station areas) which use the same frequency are geographically so located that the interference in the same frequency does not cross an allowable level. A relation between the distance R and transmission loss in free space L (R) is expressed by the equation described below (Basics of Mobile Communications: edited by IEICE).
L(R)=10 log(4xcfx80R/xcex)2xe2x80x83xe2x80x83(1)
Wherein, assuming that a carrier frequency is 5 GHz, L (R) is as follows:
L(R)=46.4+20 log(R)xe2x80x83xe2x80x83(2)
Namely, a channel having the radio wave attenuation property as shown in the equation (2) is reused at an allowable interference level. Therefore, when high-speed radio communications are to be realized in cellular radio communications, it is important to consider the problem of sending power as well as the problem of reduction of the interference in the same frequency channel.
Especially, when data transfer and radio packet transfer are to be realized with cellular mobile communications, the interference in the same frequency channel has to be reduced. That is because, when data transfer is to be performed, higher line quality is required as compared to that for speech data. For example, in a PHS system which is actually used, it is required that an error rate to speech data transfer is 1.0Exe2x88x923 or less, but an error rate lesser than this is required for data transfer. Namely, the fact described above indicates that, when the sending power is constant, a ratio of power of the desired to undesired waves is required to be made larger in the cellular radio communications, which prevents improvement in the utilization efficiency of a frequency. In addition, when high-speed data transfer is to be performed, a problem such as selective fading due to multiple scattered waves noticeably comes up, so that it is difficult to insure desired line quality.
For overcoming those problems, a cellular radio communication system using a directional antenna is known. It is known that, the directional antenna can make a delay spread smaller. Communications between a base station and a terminal station, what is called point to point communications are executed with a beam antenna by narrowing down the directivity of an antenna. Therefore, when a beam is ideally provided between two points, interference to other lines may not exist in theory, so that it is possible to execute multiple communications by dividing space even if the same sign is used at the same frequency as well as at the same time.
FIG. 15 is a view showing directions in a radio communication system having a directional antenna. In FIG. 15, the reference numerals b1 and b2 indicate base stations which use the same frequency respectively. A sector antenna having horizontal directivity of 60 degree is located in each of the base stations, and the antenna of the base station b1 covers sector cells (b1-a1, b1-a2, b1-a3, b1-a4, b1-a5 and b1-a6), while the base station b2 covers sector cells (b2-a1, b2-a2, b2-a3, b2-a4, b2-a5 and b2-a6). Terminal stations p1 and p2 are present in the sector cell b1-a1, a terminal station p3 is present in the sector cell b1-a3, and a terminal station p4 is present in the sector cell b2-a1, each of those terminal stations has a sector antenna having horizontal directivity of 60 degree, and communicates with the base station by using each antenna of the sector cells respectively.
FIG. 16A and FIG. 16B show each slot structure of frames in the base stations b1 and b2 respectively, in which the reference numerals U1 to U6 show slots for the communication line.
FIG. 16A shows how slots are allocated in the base station b1, and shows a situation in which the slot U1 is a slot allocated thereto for communications with the terminal station p1 positioned in the sector cell b1-a1, the slot U2 is a slot allocated thereto for communications with the terminal station p2 positioned in the sector cell b1-a1, and the slot U3 is a slot allocated thereto for communications with the terminal station p3 positioned in the sector cell b1-a3.
FIG. 16B shows how slots are allocated in the base station b2, and shows a situation in which the slot U2 is a slot allocated thereto for communications with the terminal station p4 positioned in the sector cell b2-a1.
If slots in a frame are randomly allocated, as shown in the slot U2 in FIG. 16A and FIG. 16B, there may occur a case where some of the sector cells oriented in the same direction of b1-a1 and b2-a1 are used at the same timing.
FIG. 15 shows the case where sector cells orient in the same direction at the same timing like the slot U2. Each directional area of the terminal station p4 and the base station b1 are shown by a dotted line. There are included not only the base station b2 but also the base station b1 within the directional area of the terminal station p4, and there is included the terminal station p4 within the directional area of the base station b1. Therefore, the terminal station p4 easily receives radio waves not only from the base station b2 which is a desired base station but also from the base station b1 which is an interference station and shows a state in which the interference becomes larger.
As a method of avoiding such a situation, there is known a method of changing the allocated slot when the interference is large. FIG. 18A and FIG. 18B, similarly to FIG. 16A and FIG. 16B, showing a state of how slots are allocated in each of the base stations b1 and b2. It is possible to avoid the antennas from orienting the same direction at the same timing by changing the allocated slot for the terminal station p4 to the slot U3.
FIG. 17 shows directions of the radio communication system in the slot U3 used by the terminal station p4. Each directional area of the terminal station p4 and the base station b1 are shown by a dotted line. As the terminal station p4 is not present within the directional area of the base station b1, interference does not occur.
As some other method to avoid such a problem, consideration has been made for channel allocation, and there has been proposed, in Japanese Patent Laid-Open Publication No. HEI 7-193857, a method of computing, when one terminal station generates a request for communications, directions which interference waves from all the channels may come from as well as a direction which a wave from the terminal station comes from, selecting one channel among channels in the order that the difference between the direction from which each interference wave comes and the direction from which the desired wave comes is closer to 180 degree, and allocating any channel first satisfying the conditions for allocation because large interference may occur if sector cells in the same direction give the same channel to terminal stations.
There is also a method of using the directivity within the perpendicular plane of an antenna as a method of suppressing the same frequency interference derived from repetition of the frequency.
FIG. 19 is a view showing directivity within the perpendicular plane of an antenna having a directional pattern correlated to each form of a service area described in a reference (xe2x80x9cBeam antenna formed for a terminal system wideband radio stationxe2x80x9d by Nomoto, Watanabe in Shingaku Giho AP88-42, 1988).
In order to reduce interference due to overreach of a frequency to other base station areas using the same frequency, a gain close to an elevation angle of zero degree drops abruptly. However, a very large antenna aperture is required to form the beam as shown in the figure, which gives rise to the problems of, for example, cost and mountability of the antenna.
FIG. 20A and FIG. 20B show a relation between interference distance, height of an antenna, and required attenuation angle. Herein, it is assumed that a term xe2x80x9coverreachxe2x80x9d indicates a phenomenon in which interference waves reach a base station from a remote base station using the same frequency, and that a distance at which the overreach happens is called as xe2x80x9cinterference distancexe2x80x9d and a difference between the elevation angle of an antenna for which waves will be within the base station area and an elevation angle thereof with which waves will reach a base station area existing within an interference distance is called as xe2x80x9crequired attenuation anglexe2x80x9d. FIG. 20A shows a relation between the interference distance and the required attenuation angle assuming that the height of the antenna in the base station is constant. In the figure, as compared to a required attenuation anglexcfx86 when an interference distance is D, a required attenuation anglexcfx86 xe2x80x98when the interference distance is longer indicated by Dxe2x80x99 may be larger, which allows design of an antenna to be easier. FIG. 20B shows a relation between the interference distance and the height of an antenna in the base station assuming that the required attenuation angle is constant. In the figure, as compared to an antenna height h when an interference distance is D, the antenna height hxe2x80x2 when the interference distance is longer indicated by Dxe2x80x2 may be lower, which allows better mountability of an antenna.
The mobile communication system using the conventional type of directional antenna requires observation of interference which may come from adjacent base stations to determine a timing of a slot for activating a directional antenna, which takes a long time and also makes the controls complicated.
In order to avoid interference from a remote base station using the same frequency by using directivity within the perpendicular plane of an antenna, a large antenna aperture as well as a certain height required for mounting a high antenna is needed, which gives rise to problems of, for example, cost and mountability of the antenna.
In a radio communication system according to the present invention, a plurality of direction-determining patterns have been prepared, a base station selects a first direction-determining pattern among the plurality of direction-determining patterns, each base station adjacent to the base station selects a direction-determining pattern different from the first direction-determining pattern, and each remote base station repeatedly uses the first direction-determining pattern, so that adjacent base stations avoid using the same antenna direction at the same time.
In a radio communication system according to the present invention, each of the direction-determining patterns has a correlation between a period obtained by equally dividing a frame for a communication line by a number of direction-determining patterns and each antenna direction, and a base station has a means for selecting one among the direction-determining patterns as well as a means for determining an antenna direction used for communications with a terminal station and allocates a communication slot used for communications with a terminal station determined as one using a particular antenna direction from slots included in the period correlated by the direction-determining pattern.
In a radio communication system according to the present invention, each of the direction-determining patterns has a correlation between a timing obtained by equally dividing a frame for a communication line by a number of direction-determining patterns and each antenna direction, and a base station has a means for selecting one among the direction-determining patterns as well as a means for determining an antenna direction used for communications with a terminal station and allocates a communication slot used for communications with a terminal station determined as one using a particular antenna direction from slots each having a timing adjacent to the timing correlated by the direction-determining pattern.
In a radio communication system according to the present invention, the base stations has a plurality of broadcasting channels each for broadcasting common information to a terminal station and selects a broadcasting channel different from those of adjacent base stations so that interference from adjacent base stations to the broadcasting channel of the base station can be avoided, and by setting a number of broadcasting channels to be the same as the number of direction-determining patterns, the base station selects one among the direction-determining patterns correlated to the selected broadcasting channel.
In a radio communication system according to the present invention with a directional antenna provided in each base station as well as with a means for setting a direction of the antenna for each communication slot and for repeatedly using radio waves at the same frequency, a number of directivity segregation is set to an integer more than 2, and a base station has a means for selecting a directivity segregation number as a natural number smaller than the number of directivity segregation, each of base stations adjacent to the base station has a means for selecting a directivity segregation number different from the first directivity segregation number, and each of remote base stations can repeatedly select the first directivity segregation number; and for the purpose of allocating a communication slot to a terminal station, a base station has a means for obtaining sector numbers as natural numbers into which an angle of a radio wave to a terminal station is classified; a means for obtaining, by combining the sector number with the directivity segregation number of the base station, allocation-target slots different from each other; and a means for allocating slots to terminal stations around each of the allocation-target slots.
In a radio communication system according to the present invention with a directional antenna provided in each base station and a means for setting a direction of the antenna for each communication slot and for repeatedly using radio waves at the same frequency, a number of directivity segregation is set to an integer more than 2, and the base station has a means for selecting a directivity segregation number as a natural number smaller than the number of directivity segregation, each of base stations adjacent to the base station has a means for selecting a directivity segregation number different from the first directivity segregation number, and each of remote base stations can repeatedly select the first directivity segregation number; and for the purpose of allocating a communication slot to a terminal station, a base station has a means for obtaining allocation-target slots different from each other by combining an angle of a radio wave to a terminal station with a directivity segregation number of the base station; and a means for allocating slots to terminal stations around each of the allocation-target slots.
In a radio communication system according to the present invention, the base station has a plurality of broadcasting channels for broadcasting common information to terminal stations, and the base station avoids, by selecting a broadcasting channel different from those of adjacent base stations, interference from the adjacent base stations to the broadcasting channel of the base station, and by setting a number of broadcasting channels to be the same as the number of directivity segregation, the base station selects the directivity segregation number correlated to the selected broadcasting channel.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.